The present invention relates to a device and a method for holding substrates in microlithography, such as writing of, measuring of or printing from patterns on large flat substrates, in particular large-area photomasks for production of visual display devices such as TFT-LCD screens for portable computers. To the extent that other implements such as the display glass panels themselves, printed circuit boards or semiconductor reticles or wafers pose the same problem of maintaining the geometrical precision across several types of equipment, the invention can be applied there too.
In microlithography, and especially in microlithographic writing of large areas, as is known from e.g. EP 0 467 076 by the same applicant, flatness errors frequently occur due to the substrate not being totally flat during the writing process.
It is known in the art that a flat workpiece requiring the utmost precision is deformed when it is clamped to a stage or workpiece holder. For the discussion we consider a flat glass plate with the size 600xc3x97800xc3x978 mm, such as a photomask for production of TFT-LCD display screens. The xy precision in production plate needs to be of the order of 0.3 xcexcm (3 sigma or 99.7% confidence) everywhere when compared to an ideal coordinate grid. In a metrology situation the precision should, if normal metrology standards are to be maintained, be three times better, i.e. 0.1 xcexcm.
If the glass plate is placed on a horizontal table, even minute deviations from flatness in the table affect the geometrical precision of a pattern written on the glass. Micron-sized particles between the glass and the table also make a measurable deformation of the glass. Therefore a metrology system, and to a lesser extent a pattern generator and an exposure station using the mask, must have a stage that is built to extreme precision and kept meticulously clean. A fingerprint on the stage can ruin the precision.
However, even with a perfect stage the precision is limited by the flatness of the glass, since large glass plates are compliant enough to make the surface resting on the stage follow the stage surface, fully or partially. The problem becomes intractable when one realises that different manufacturers support the plates differently and that in different types of equipment different types of clamping is required or technically desirable. An example is an exposure station where the mask has to be clamped by the edges since no supports are allowed to obstruct the light transmission through the mask within the patterned area. Furthermore there may be different preferences among manufacturers and different technical requirements on the orientation of the patterned glass plate: horizontal with the pattern up, with the pattern down or standing or hanging more or less vertical.
It is known in prior art to support the glass on three points. Unfortunately this works only for small glass plates such as semiconductor reticles which are stiff enough not to bend excessively under the gravitational force.
A deflection, e.g. caused by the attraction of gravity, results in a expansion on the bottom side of the substrate and a compression on the top side, as is shown in FIG. 2. Hereby there is an angle xcex2 present between the intended path for the laser rays, perpendicular to the substrate surface, and the actual path. The error xcex5 at the surface of the substrate between the position intended to be hit by the laser light and the position actually hit, is then:                     ϵ        =                              t            2                    ⁢          β                                    (        1        )            
where t is the thickness of the substrate.
This expression is approximately the same as:
xcex5=xcex94hxc2x7t/2axe2x80x83xe2x80x83(2)
where xcex94h is the deflection of the substrate and a is half the length of the substrate (i.e. the length is 2a). In a typical situation t could be 8 mm, a 400 nm and xcex94h 1 mm. The error would in this case be 10 xcexcm, which is a most significant error in the intended application.
The flatness error is to a large extent a linear problem, which relatively easily can be compensated for electronically. The remaining part, which is normally about 20% of the total error and therefore most significant, is however non-linear, which makes it difficult to deal with.
The flatness error also give rise to a problem with keeping the focus of the laser beam, while the depth of the focus is limited.
It is therefore an object of the present invention to provide a device and a method for holding substrates in microlithography whereby the flatness error problem is at least reduced.
This object is obtained with the invention such as it is being defined in the attached claims.
The present invention relates to a method and device for maintaining geometrical precision in the case of a non-ideal support structure holding a non-flat unknown substrate, oriented arbitrarily to the gravitation force, whereby the deflection of the substrate is neutralised with a pressure difference between the different sides of the substrates.
In a companion application the method to completely control both clamping and shape-induced errors, by characterising the glass plates and the equipment independently and compute the distortion for the combination is described, i.e. it uses a mathematical model to correct for actual errors in existing equipment. Compared to the present invention the method of the companion application is applicable without the requirement that the equipment is specially built for this purpose, but on the other hand the present invention is simpler and easier to use.
Although the invention is applicable to different sizes of plates of glass and other materials its first application is to photomasks in large-area lithography. The plates with size up to one meter are made from fused silica for reasons of thermal expansion and polished flat. The alternative to the invention, and to the invention in the companion application, is to make plates that are flat enough to be considered ideal. Such plates would be incredibly expensive, while the present invention does not require flatness above that needed for other reasons, such as focus control. Furthermore focus control can be made better with the present invention.